This invention relates to a solid-state imaging device, and more particularly to a solid-state imaging device having a miniaturized structure.
The configuration of a conventional typical solid-state imaging device when viewed from the plane is shown in FIG. 1. In this imaging device, photosensitive elements 2 and 3 formed on the surface of a semiconductor substrate and isolated in an X-direction and in a Y-direction by device isolation layers 1 indicated by slanting lines are arranged one after another in a train form. Adjacently to photosensitive element trains comprised of a plurality of photosensitive elements, a charge transfer channel 10 comprised of an impurity layer is provided. On the charge transfer channel 10, transfer electrodes 4, 5, 6 and 7 are successively formed in a repeated manner. These transfer electrodes are of well known superposed electrode structure, wherein the transfer electrodes 5 and 7 are a first layer electrode, and the transfer electrodes 4 and 6 are a second layer electrode. Generally, four-phase transfer pulses .phi..sub.1 to .phi..sub.4 are applied to these transfer electrodes to carry out the transfer operation. Namely, transfer pulses .phi..sub.1 to .phi..sub.4 are applied to the transfer electrodes 4, 5, 6 and 7, respectively.
Between the photosensitive sections 2 and 3 the charge transfer channel 10, charge transport channels 11 and 12 are provided, respectively. Between respective photosensitive sections 2 and 3, interconnection portions 24 connecting adjacent electrodes of the same phase are formed.
The operation of the imaging device thus constructed will now be described.
When a high voltage readout pulse is applied to the electrode 4, the charge transport channel 11 is turned ON. As a result, signal charges are transported from the photosensitive element 2 to the portion below a corresponding electrode of the charge transfer channel 10. Similarly, when a high voltage readout pulse is applied to the electrode 6, the charge transfer channel 12 is turned ON. As a result, signal charges are transported from the photosensitive element 3 to the portion below a corresponding electrode of the charge transfer channel 10. Thereafter, when well known four-phase pulses are applied to the transfer electrodes 4 to 7, signals are transferred in a lower direction of plane surface of paper.
It is to be noted that, in an ordinary operation, in accordance with the interlace operation in the well known television system in the first field (first charge readout), signal charges in the photosensitive elements 2 and 3 are added, and in the second field (second charge readout), signal charges in the photosensitive element 3 and the next photosensitive element 2 (represented by reference numeral 2' in FIG. 1 for convenience) are added. The signal charges thus added are read out. Such a readout system is generally known as the storage readout mode.
The configuration in cross section cut along the lines X.sub.1 -X.sub.2 of FIG. 1 is shown in FIG. 2. A p-well 13 is formed on an n.sup.- -type substrate 14. Within the region isolated by device isolation layers 1, there are formed a photosensitive element comprised of an n-type impurity layer 2-a and a p.sup.+ type impurity layer 2-b covering the surface thereof, and a charge transfer channel 10 through a charge transport channel 11. An insulating film 15 is deposited on the substrate, and a transfer electrode 4 is formed in the insulating film above the charge transfer channel 10 and the charge transport channel 11.
FIG. 3 is a diagram showing the distribution of potentials at respective portions of the channel from the n-type impurity layer 2-a up to the charge transfer channel 10 via the charge transport channel 11 wherein reference numeral 23 represents a potential on the device isolation layer, reference numeral 22 a potential on the photosensitive element in a charge empty state, reference numeral 18 a potential on the charge transport channel in an ON state, reference numeral 19 a potential on the charge transport channel in an OFF state, reference numeral 20 a potential on the transfer electrode 4 in an ON state, and reference numeral 21 a potential on the transfer electrode in an OFF state.
Assuming now that the width of the transfer electrode 4 is W and the length between the photosensitive element 2 and the charge transfer channel 10 is L (see FIG. 1), let consider the case where the length L is short. In this case, when a large quantity of charges are stored into the n-type impurity layer 2-a of the photosensitive element by the short-channel effect well known as the phenomenon that the isolation characteristic between the photosensitive element and the transfer channel is deteriorated when the transfer electrode is in an OFF state, there occurs the phenomenon that a portion of the stored charges overflows into the charge transfer channel 10. This state is represented by reference symbol OFL in FIG. 3 as the overflow phenomenon that when the charge transport channel is in an OFF state and a potential thereon is thus represented by reference numeral 18, signal charges produced in the photosensitive element 2 climb over the potential barrier 18 to flow out into the charge transfer channel 10.
In order to eliminate such an undesired phenomenon, it is required to set the length L between the photosensitive element 2 and the charge transfer channel 10 to a value longer than a sufficiently large value (e.g. 1.5 .mu.m).
On the other hand, the length L varies by .+-..DELTA.L (e.g., .+-.0.5 .mu.m) by errors in manufacturing the solid-state imaging device, e.g., various tolerances or alignment deviations, etc. Accordingly, when such errors in manufacturing are taken into consideration, the length L must be set to L+.DELTA.L.
From the above-mentioned circumstances, the width of the electrode cannot help being set to a value far larger than the width primarily required for transfer. This makes it difficult to ensure a sufficient photosensitive element area, resulting in lowered sensitivity and obstructions to the device miniaturization.